The technique of soil consolidation has been used for the static retrofit of existing structures for several years. Soil consolidation is ideally suited for solving foundation problems in areas of tight access, low overhead or difficult geology conditions. A typical method of soil consolidation is to drill a bore by rotating a rotary drill bit by means of a tube string, opening a free end of the string and inserting a ball that gravitates onto a seat in the drill string located adjacent to the drill bit. Thus a check valve is created shutting the channel to the rotary drill bit and allowing jet grouting of the soil adjacent to the hole during retraction of the string and the drill bit. When the soil to be consolidated includes big boulders (e.g., having diameters of 0.3 to 1.0 m) the known method becomes ineffective in terms of penetration speed.
When that kind of boulder-containing soil is to be consolidated it is often necessary to use a top hammer equipment wherein the hammer impacts on a sealed drill string which transfers the resulting shock waves to a percussive drill bit. When the drill bit has reached its predetermined position down into the soil, jet grouting is commenced at 300 to 500 bars of internal pressure. However, the shock waves will eventually impair the function of the seals mounted in every drill string joint, whereupon the grout will leak and abrade holes in the expensive drill tubes. Also, the leakage will result in the jet grouting being performed at a lower pressure than intended. As soon as the leakage is discovered the drill tube is exchanged.
Disclosed in U.S. Pat. No. 5,219,247 is a soil consolidation apparatus utilizing an air-driven down-the-hole hammer. However, that apparatus requires the use of at least two pressurizing means, i.e., one compressor to pressurize air and one pump for jet grouting. Furthermore, the air has to be pressurized to a high level by the compressor for lifting the cuttings, and thus the soil surrounding the hole will be eroded by the high-pressure air.